The present invention relates to a spinal disc prosthesis to replace a damaged or degenerated spinal disc in a spinal column of a human.
U.S. Pat. Nos. 5,017,437 and 5,534,030 disclose typical spinal disc prostheses to replace a damaged or degenerated spinal disc in a spinal column of a human. The discs disclosed in these patents include a pair of rigid plates adhered to opposite surfaces of a body of elastomeric material. In U.S. Pat. No. 5,534,030, the opposite surfaces of the body of elastomeric material to which the rigid plates are adhered extend at an angle to each other as they extend across the disc. The rigid plates which are adhered to the elastomeric material are not wedge-shaped, but the spinal disc comprising the elastomeric core and the rigid plates is generally wedge-shaped.
The disc when in use is positioned between adjacent vertebrae, and the rigid plates have bone ingrowth material for enabling bone to adhere or fuse to the rigid plates. The disc is subject to forces which act in the spine including compression forces due to loads on the spine, shear forces due to bending of the spine, and torsional forces due to twisting of the spine. These forces can be applied simultaneously to the disc. These forces may cause the rigid plates to separate from the body of elastomeric material. Such separation would be detrimental to the proper functioning of the disc.
Also, it is desirable to control relative displacement of the rigid plates when in use to minimize the possibility of spinal instability. An excessive amount of relative displacement would not be desirable.
It has been discovered that the maximum forces acting on a spinal disc, and particularly the maximum forces tending to separate the rigid plates from the body of elastomeric material, can be reduced and the relative displacement of the rigid plates can be effectively controlled by constructing the spinal disc so that the disc comprises the following:
1. an elastomeric core having upper and lower surfaces which are parallel to each other,
2. an upper rigid plate having a first surface affixed to the upper surface of the core and a second surface for adherence to a vertebra, which second surface is inclined relative to the first surface, and
3. a lower rigid plate having a third surface affixed to the lower surface of the core and a fourth surface for adherence to a vertebra, which fourth surface is inclined relative to the third surface,
4. the second and fourth surfaces being inclined relative to each other to give the disc a wedge shape.
When the spinal disc is in use between adjacent vertebrae, the second surface is inclined away from the first surface as the second surface extends from a posterior portion of the spinal disc toward an anterior portion of the spinal disc. Also, the fourth surface is inclined away from the third surface as the fourth surface extends from the posterior portion of the spinal disc toward the anterior portion of the spinal disc.